Processing of scrap metal

ABSTRACT

Solid state processing of scrap metal cuttings. The cuttings are compacted, conveniently at room temperature, and then jacketed in a substantially air tight jacket. Matter, such as graphite, which gives off a reducing gas is also introduced in the jacket which is then heated so that the reducing gas reduces the surface oxides present in the scrap. The jacketed mass is then hot worked, e.g., by rolling to form a homogeneous mass.

[451 Nov. 27, 1973 Emme@ States Patent Cacace et al.

Patented Nov. 27, 1973 3,774,289

PROCESSING OF SCRAP METAL This invention relates to the processing ofmetal scrap.

The disposal of metal scrap in the form of cuttings such as turnings,borings, sawings and the like, as well as of off cuts from sheet-metalsheets is a constant problem. These cuttings have a high surface area tovolume ratio and are very prone to oxidation. They are bulky anddifficult to handle. Furthermore, a high percentage of the scrap metalis lost when cuttings of this nature are remelted.

These problems are particularly marked in the case of steel scrap if forno other reason than that steel forms a very high percentage of thedifferent metals used in industry.

A number of proposals have previously been made to render the re-use ofmetal scrap possible without remelting it. In one process, a mass ofcuttings is consolidated and subsequently hot forged into a billet andthereafter rolling the billet into a product. Some prior proposalssuggest that it is possible to consolidate a mass of cuttings andsubsequently press these cuttings in a die to form a pressed billet. Allof these proposals produce a product of uncertain metallurgical qualityand of generally poor surface appearance. The difficulties involved havebeen such that, to the applicants knowledge, none of the processes is incommercial use.

The reason for the occurrence of these problems is almost certainly thepresence of oxide on the surface of the scrap. The term oxide includesrust for the purposes of this specification. During the course ofexperiments connected with the present invention, it was found that nomatter how clean the scrap was before use, i.e. clean of oxides and rustas well as of foreign matter such as oil, grease and the like, thedifficulties persisted. The conclusion drawn was that, even if theconsolidated mass was heated in a reducing atmosphere prior tohotworking, sufficient oxidation occurs during the transference of theheated mass to the press to render the process in that form unworkable,due to the oxide inclusions occuring in the final product.

lt is an object of the present invention to provide a process in whichthese difficulties are obviated.

According to the invention, there is provided a process for producing asubstantially homogeneous product from scrap metal cuttings, includingthe steps of:

a. compacting a quantity of the scrap metal cuttings,

at a temperature below that at which a significant degree of oxidationcan take place, into a mass having a density of at least about 50percent of the density of a homogeneous mass of the same metal: b.jacketing the mass in a substantially airtight jacket; c. causing areducing gas to be present within the jacket and, by applying heat,causing the gas to reduce surface oxides present on the scrap; and

d. hot-working the de-oxidised jacketed mass to form the homogeneousproduct.

The compaction can be carried out at room temperature. The degree ofcompaction is desirably as high as economically possible without beingso high that the reducing gas cannot penetrate the interstices betweenthe cuttings. Also, it has been found that some degree of compaction isnecessary.

This is so because it is obviously in many circumstances desirable thatthe jacket should be as thin as possible. lf the cuttings are notcompacted, a thinwalled jacket frequently ruptures under the pressure ofthe subsequent hot-working, rendering the billet useless. It isconsidered that for a thin jacket, 60 percent compaction is about theminimum degree of compaction, from the point of view of economics,although 5 0 percent compaction is still workable. However, percent is agood working compaction from the point of view not only of economics butalso for ensuring that the compacted mass is strong enough to preventthe rupture of the jacket.

lt is a feature of the invention that the cuttings can be compactedbefore being jacketed. Furthermore, this step can be carried out in acommercially available briquetting machine. If a large billet'isrequired, a number of briquettes can be introduced into the jacket andagain compacted before the jacket is closed. One of the incidentaladvantages of the process is that the jacket can be in the form of apipe which can fit snugly into the pressing die. The presence of thejacket renders die wear negligible. l

The reducing gas can be introduced into the jacket in any suitablemanner. One of the most convenient methods of achieving this is tointroduce a solid substance into the jacket before the scrap is insertedtherein. This substance then combines with another substance in thejacket to evolve the reducing gas. A very convenient such substance iscarbon. This can be in the form of graphite powder or, when medium orhigh-carbon steel scrap is used, there is sufficient CO generated by thedecarburisation of the steel when heated to reduce all the oxide.

It is desirable, if not essential, to allow excess reducing gas toescape from the jacket. It has been found during the course ofexperiments carried out in connection with this invention that if thejacket ends are simply folded over the compacted mass or beaded over endplates inserted in the jacket ends, so that the mass is completelycovered, this will normally provide a sufticient degree of protectionfrom oxidation, while allowing the excess reducing gas to escape. Thisis a particularly convenient method in the case of a thin walled jacket.For a thick walled jacket, it may be desirable to weld a plug in eachend of the jacket. ln this case it will be necessary to provide a venthole in the plug or jacket.

Another very important feature of the present invention is that thejacket can be converted, in the hotworked product, into a corrosionresistant external layer. To this end, the invention provides that thejacket be of corrosion resistant metal, for example stainless steel. lthas been found, due to the very even degree of compaction obtainable bythe use of briquettes, the jacket remains on the outer surface of thecompacted mass during hot-working, and in the final product forms anevenly spread external layer over the final product. Clearly the jacketmust be thick enough to provide this result.- Equally thecross-sectional shape of the product must not be too complex. For barsof round or square section, however, the thickness of the outer layer issurprisingly constant.

It is envisaged that a step of cleaning the cuttings prior to compactionwill normally be carried out. Furthermore, the cuttings will normally bereduced to chip but is particularly convenient for cuttings in the formof turnings, borings, sawings and the like.

Another feature of the invention is that the process can be applied tohot-working in the form of rolling. Because of the very high localpressure applied by the rolls to a billet, it is necessary that thebillet be reasonably strong. This can be achieved by hot compaction inthe case of a billet which is not jacketed, but the jacket, especiallyif it is of relatively large wall thickness, renders this hot-compactionunnecessary and even a disadvantage because of the increased danger ofoxidation.

A further feature of the invention is that the process can be applied tohot-working in the form of extruding the material. With this processthere is hot compaction of the jacketed billet in a conventionalextrusion press and thereafter the billet is extruded into any plain orhollow section as tube. In such a case the jacket wall need not be oflarge thickness as it is supported by the walls of the extrusion presscylinder and this will make the production of steel more economical.

In an embodiment of the invention, steel turnings in their mosteconomical form are used. In this form they have undergone noconsolidation of compaction at all and are consequently very bulky andspringy. The steel turnings are reduced in size in a chipping machineinto lengths of about 3 cm or less. The chips are then cleaned fromoil-emulsion, grease, soil and rags by washing with organic solvents.Alternatively they may be steam degreased, and hot-air dried. If theonly impurity is the oil-emulsion, they may simply be centrifugallycleaned.

The chips are then converted into briquette form in a commercialbriquetting machine. Prior to briquetting, very fine grain powderedgraphite in a proportion of 0.1 percent to 0.05 percent by weight ofgraphite to chips, is added to the chips. The briquettes are then placedin a close fitting tube and pressed together at room temperature. Thedensity of the compacted chips in the tube is desirably at least about80 percent of the weight of a solid steel compact of the same size theequivalent density of the briquettes being at least about 60 percent.The pressing operation is carried out so that pressing is achieved bytwo rams which press directly onto the chips from either end of thetube, the tube being supported in a die. A thin graphite film can beplaced on the inside and outside of the tube to provide lubrication.

lt is also possible to insert the chips loose into the tube and then tocompact them. In this case several operations of inserting chips andthen pressing are required, the pressing operations prior to the finalone being carried out at a pressure lower than that used in the finalone.

The billet is then heated up so that it attains a uniform temperature ofl,250 C, by any convenient means. A protective atmosphere can be usedwhile heating if required. During heating the graphite in the tube formscarbon monoxide which reduces all surface oxide films and rust presenton the chips and ensures that any subsequent hot-working such as hotrolling is carried out under a reducing atmosphere inside the tube. Theformation of CO from C and CO2 or C and O2 results in two molecules ofCO from each molecule of CO2 or O2. As a result the pressure inside thetube is always higher than outside the tube, ensuring that a reducingatmosphere is always present within the tube.

It is very common to find that steel turnings are of medium or highcarbon steels. In the case of such steels it is possible to omitgraphite powder altogether. In this case, the steel decarburises thusproviding the source of carbon necessary for the production of CO.

The billet can be of any convenient size. Up to the present time, abillet of 180 mm in diameter and 1,000 mm in length, weighing about Kgm,is the maximum size produced by the applicants.

BRIEF DESCRIPTION OF THE DRAWING FIG. l is a sectional view of theproduct of scrap metal cuttings wherein the jacket ends have been foldedover end plates at opposite ends of the compacted cuttings, and

FIG. 2 is a sectional view of the product of scrap metal cuttingswherein the jacket ends are welded to plugs at opposite ends of thecompacted cuttings.

EXPERIMENT l In a typical experiment to observe the reducing action ofcarbon monoxide on rust and oxide films on steel turnings, the followingprocedure was carried out:

Eight specimens were prepared consisting of four briquettes preparedfrom heavily rusted steel turnings and four briquettes prepared fromnon-rusted turnings. Two of each batch contained graphite powder mixedwith the turnings in the proportion of 0.1 percent by weight. Thebriquettes were l0 Cms in diameter and 6 cms in height. The briquetteswere then placed into tight fitting cylindrical containers and thecontainers were then sealed. A hole of 2mm in diameter was drilled inthe top of each container. Four of the containers were selected; thefirst containing the rusted swarf plus graphite; the second containingrusted swarf with no graphite; the third containing non-rusted swarfcovered with the well-known blue oxide film plus graphite, and thefourth containing the same as the third except with no graphite. Thesefour speciments were placed into a small furnace and heated gradually upto 1,200 C over three hours.

They were then removed from the furnace and the small holes in thecontainers sealed by welding. The containers were then rapidly cooled byimmersing them in a bath of oil. The other four containers containingsimilar contents were placed into the hot furnace at l,200 C and heatedfor one hour. The same procedure was observed for cooling as for thefirst four containers. All eight containers were opened. Very brightsteel turnings were observed in all of them, indicating that completereduction had occurred.

The containers with graphite contained turnings where were less pliablein bending and more brittle than the turnings with no graphiteindicating that carburisation had occurred to a slight extent. The steelturnings had sintered together to some extent; and it was possible tosaw through the sintered briquettes with a power saw. On crushing asawed slice of briquette with an hydraulic press the turnings at theinner centre of the slice were as bright as those at the exterior of thebriquette.

Experiment l indicates the state of steel turnings just prior to hotworking. Completely reduced surfaces on the turnings ensure that soundwelding between surfaces will occur when hot worked, and the reducingatmosphere that no oxide inclusions can possibly occur, during hotworking.

Experiment 2 ln a typical experiment to produce a section the followingprocedure was observed. Briquettes pressed to a density of 65 percentwere produced from prepared turnings, mixed with 0.1 percent graphite.The briquettes were l cms in diameter and 8 cms in height and had a meanweight of about 4 Kgms each. Four of these briquettes were placed in atube of` inner diameter 10.4 cms and exterior diameter 1l cms. The tubewas 36 cms in length and had a thin film of graphite painted on theinterior and exterior of the walls of the tube.

The filled tube was placed in a steel die of inner diameter l 1.2 cms,thickness of 7 cms and length of 35 cms. Two close fitting rams wereplaced in the open ends of the tube so that they pressed directly on thecolumn of swarf in the tube. A primary pressing was applied and then alonger ram was placed in the bottom end of the tube so that a floatingdie action is achieved when giving the maximum pressure to a density of80 to 85 percent. The tube was then pressed out of the steel die. Thetube ends were sawed off flush with the surfaces of compacted steelturnings and two discs of steel of similar thickness to the tube werewelded on the open ends of the tube.

A hole of 2 mm in diameter was drilled in each of the steel discs.

The billet was heated in a conventional reheating furnace up to l,200 Cover three hours. A blue flame was observed burning at each of the holesdrilled in the disc, indicating the expulsion of carbon monoxide fromthe interior of the tube. The billet was then rolled in a conventionalthree-high merchant mill with a primary passage designed for a squarebillet of dimensions l 1.3 cm square. A reduction of 2 mm on thediagonal was effected through each of the first eight passages of themill. The billet was reduced down to a rectangular section of mm lO mmand a length of about l5 m. The surface appearance was perfect and nodifference between conventionally produced steel and the steel producedby the present method could be visually observed.

Sections of length 30 cms were cut along the entire length of the barand were subjected to tensile tests. Pieces of bar in length and alsocross-section were polished and etched and were subjected tometallographical examinations and photographs. The micrographs indicatevery clear junctions along which the swarf particles have weldedtogether. No oxide inclusions are visible.

Table l indicates the physical properties of lengths of bar:

TABLE l Bar Approx. Ultimate No. elongation over Yield Tensile 2 inchesat fracture stress Stress l l0.40 54.50 79.50 2 l 1.05 52.50 75.40 J10.04 56.00 77.90 4 l l.l0 54.00 77.00 5 8.90 62.00 79.40 h 8,85 ,62.7580.30

Experiment 3 A stainless steel tube of outside diameter 110 mm,thickness 6.35 mm and length 360 rnm was employed and the same procedureas in Experiment 2 was observed except that loose swarf and notbriquettes was inserted and pressed.

When the tube was sufficiently filled with compacted turnings the fullpressure was applied and the billet removed. The same rolling procedurewas observed as in the case of Experiment 2 except that an intermediateheating of the billet was necessary as the stainless steel exteriorcooled more rapidly than the former mild steel exterior.

The rolled product was perfect and on polishing and etching across-section of the 3.5 cms X l cm bar, the outer stainless steeljacket could be distinctly discernible as an unetched uniform unbrokenwhite layer of about 2 mm to 3.5 mm which was perfectly welded onto thecore.

Experiment 4 The same procedure as in Experiment 2 was 0bserved exceptthat the billet was hot pressed to about 100 percent compaction prior torolling. However, during hot pressing oxidation occurred to a slightextent and the surface of the rolled product was not as satisfactory asthat in Experiment 2.

Experiment 5 percent compaction briquettes were placed in a tube withoutpressing similar to that employed in Experiment 2. The billet was thenrolled. A good rolled product resulted.

Experiment 6 Experiment 2 was repeated with medium carbon steel andwithout the addition of graphite. Results from the Tensile Test indicatebetter elongation values.

TABLE 2 Bar Yield Point elongation Ultimate tensile Tons/sq. in. l.5"stress tonslsqjn. l 3l l7 39 2 28.5 l2 34 Many experiments similar tothose described in Experiments 2 to 6 were carried out. No section thatwas tested in tension gave tensile properties inferior to those given intable l and table 2. These physical properties are representative of ahigh tensile steel class of section.

This new steel making method lends itself` to a very economicalindustrial process of producing high tensile steel.

The method is unique for the production of stainless steel-clad hightensile steel. Many applications especially in the chemical industrieswhere costs are prohibitive if stainless steel sections are employedcould make use of stainless steel-clad steel sections. Stainless steelcladding would represent about l0 to 2O percent of' the weight of theclad steel.

For the purposes of this specification a homogeneous product should beunderstood to mean a product in which the constituent pieces of metalare welded or fused together. l

We claim:

l. A process for producing a substantially homogeneous product fromscrap metal cuttings on which surface oxide is present and whichcuttings include residual carbonaceous material therein as an originalconstituent, including the steps of a. compacting a quantity of thescrap metal cuttings substantially free of extraneous carbonaceousmaterial at a temperature below that at which a significant degree ofoxidation can take place, into a mass having a density of at least about50 percent of the density of a homogeneous mass of the metal;

b. jacketng the mass in a jacket which substantially tially all of thecuttings are steel.

1k k Ik Ik

2. A process according to claim 1 in which substantially all of the cuttings are steel. 